What are my rights for false collection notice past the thirty days?

I received a notice from AFNI Collections stating that I owe money on an account I never had. I did not see the letter until today, it must have gotten lost in the mounds of junk mail I receive. Can I still write a letter disputing it, even though it is past the thirty days? What course of action should I take to resolve this. I have read many complaints about fraud with this company.